Time of flight imaging is used in a number of applications including range finding, depth profiling, 3D imaging (e.g. LIDAR), and medical imaging techniques. Direct time of flight measurement involves measuring the length of time between emitting radiation and sensing the radiation after reflection from an object. From this, the distance to the object can be determined. In specific applications, the sensing of the reflected radiation may be performed using a Single Photon Avalanche Diode (SPAD) array. SPAD arrays have been used as solid-state detectors in imaging applications where high sensitivity and timing resolution are required.
A SPAD is based on a p-n junction device biased beyond its breakdown region. The high reverse bias voltage generates a sufficient magnitude of electric field such that a single charge carrier introduced into the depletion layer of the device causes a self-sustaining avalanche via impact ionization. The avalanche is quenched, either actively or passively, to allow the device to be “reset” to detect further photons. The initiating charge carrier may be photo-electrically generated by a single incident photon striking the high field region. This single photon detection mode of operation is often referred to as “Geiger Mode”.
Time to digital converters are sometimes used in time of flight imaging applications to convert an event arrival time to a time-stamped digital output. Often system requirements require the time to digital converter timing resolution to be over that of a single clock cycle. One example time to digital converter configuration is set forth in U.S. Pat. Pub. No. 2015/0041625 to Dutton et al., which is assigned to the present Applicant and is hereby incorporated herein in its entirety by reference. This time to digital converter includes a sample module operable to sample an input signal at multiple different instances of time. A transition detection module, formed of comparison elements, processes the sampled input signal at successive time instances to detect transitions in the input signal in terms of time. An output module generates detected transitions in the input signal on multiple parallel outputs.
Despite the existence of such configurations, further enhancements in time of flight imaging systems may be desirable in some applications.